Fight, flight or stab
When running into combat you'll be given three main options. Fight, stealth, or assassinate.
Of course, only assassins will be given the assassinate option, and their success rate will depend on their skill - though, if you can pull it off, it's definitely the best option.
Stealth is another good option, allowing you to bypass enemies, but if your assassin is weak, this won't go well.
Straight up combat is the most definitive option, but has some intricacies we'll go over here. It's also pretty likely to leave you damaged - yikes.
With their back turned
When going into combat you'll have the best luck if the enemies have their back turned. You'll be able to see where they're facing before entering a room.
When they're facing away you can get a clean surprise attack on them. Use your purple attack which often does more damage, as you'll have a higher chance of a critical strike.
In these situations it might be better using a combat focused character instead of a low-health assassin.
For whom the bell tolls
When staking out rooms before entering, be wary of bells. When enemies stand next to these, they are likely to ring them and summon more enemies.
Making things worse, these summoned enemies can appear on both sides of you, surrounding your character and allowing enemies two hits on you at a time. Ouch.
However if you use an assassin, stealth or assassination attempts should lure enemies away from the bell, even should you fail.
Ready to die
If you want to get three stars in each stage, you need to keep your full team alive. But that is easier said than done.
Sometimes, you can barely scrape through a stage with luck, and you'll need to risk it all. For example, sending a skilled assassin into a crowded room in hopes they can assassinate everyone and not fight and die.
You can get lucky, and you can also get unlucky, but it just means that you might succeed once you retry.